The present invention relates to a photopolymerizable mixture or composition containing a polymeric binder, a polymerizable compound, a photoinitiator and a leuco dye.
Mixtures of the above-indicated composition are known in the art. The addition of a leuco dye has the effect that, after exposure, the mixture exhibits a visible contrast between exposed and unexposed areas, since leuco dyes are oxidized during or after exposure to form the corresponding dyes. Mixtures of this kind are described, for example in German Offenlegungsschrift No. 31 31 448 and in European Patent Application No. 230 941.
Due to the fact that leuco dyes are slowly oxidized even in the dark, these mixtures tend to discolor during dark storage. This tendency is increased when photooxidizing agents, for example, trihalogenomethyl compounds which split off halogen radicals upon exposure, are added to obtain an intensification of contrast. A number of stabilizers, for example, sulfur compounds, phenols and other reducing agents have therefore been recommended for photopolymerizable mixtures and materials of this kind.
German Offenlegungsschrift No. 35 34 527 describes thiuram disulfide for use as a stabilizer. Similarly to thiourea, thiuram disulfide is a moderate stabilizing agent. These compounds, however, have the disadvantage of reacting with a copper surface, which gives rise to annealing tints, staining upon exposure and problems in subsequent electroplating processes. In addition, light sensitivity is reduced by thiuram disulfides.
U.S. Pat. No. 3,042,515 mentions stabilizers comprising phenols, such as resorcinol or hydroquinone. In most cases, however, the effect of these compounds is inadequate.
German Offenlegungsschrift No. 29 41 846 describes 2,4-dihydroxybenzaldoxime as a reducing stabilizer, which has an improved stabilizing action. But even when this compound is added, shelf life is still insufficient and, in addition, the adhesion of the layer to copper is impaired.
European Patent No. 63 304 teaches addition of particular epoxy compounds to photopolymerizable mixtures for thermal post-hardening. Since these compounds act as crosslinking agents they must necessarily contain two epoxy groups. They are also added in relatively large amounts of 1.5 to 10, preferably 2 to 6, percent by weight.
U.S. Pat. No. 4,146,453 describes a photopolymerizable composition comprising ethylenically unsaturated compounds and certain particularly active photoinitiators, e.g., benzoin or benzoin derivatives. With respect to these particularly active photoinitiators, the composition is stabilized against premature polymerization by the addition of certain epoxy compounds. Moreover, U.S. Pat. No. 4,171,977 discloses the use of certain epoxy compounds as thermal stabilizers to inhibit premature polymerization in a photopolymerizable composition. Neither of these, however, address the problem of leuco dye stabilization.